1. Field of the Invention
The present invention relates to an exposure apparatus and a method of manufacturing a device using the exposure apparatus.
2. Description of the Related Art
An exposure apparatus is employed to manufacture a semiconductor integrated circuit having a fine pattern formed on it. The wavelength of light used in the exposure apparatus is shortened to keep up with pattern miniaturization. At present, various attempts are being made using soft X-rays. All substances have strong absorption in the soft X-ray wavelength range of 1 to 100 nm. Therefore, for this wavelength range, no practical transmissive optical element which uses refraction is available, unlike the visible light range, and an ultrathin-film filter or a reflector on which a multilayer film is formed is used as an optical element. For example, a multilayer mirror capable of obtaining a reflectance of about 70% as a normal-incidence reflectance has been fabricated for wavelengths around 11 nm and 13 nm, and an exposure apparatus including a reflective optical system having this multilayer mirror has been proposed. When a thin-film filter or a multilayer film is used, dust which adheres to its surface poses a serious problem because all substances have strong absorption in the soft X-ray range. Because soft X-rays are absorbed even by the air, the optical path must be evacuated to a vacuum. However, a very small amount of, for example, oil flows backward from a normal exhaust system, so carbon dust is formed on the surface of an optical element upon irradiation with soft X-rays in that state. This phenomenon is called carbon contamination. This decreases the transmittance or reflectance of the soft X-rays. In an exposure apparatus which uses soft X-rays, a wafer coated with a photosensitive polymer called a resist is loaded into a vacuum enclosure (vacuum chamber) in exposure, and a very small amount of gas generated by the resist may also produce carbon dust. A method which is most commonly used as a method of cleaning the substance surface is a wet cleaning method in which a treatment that uses a chemical solution and a rinse that uses pure water are combined. This cleaning method is very widely employed in, for example, a process of manufacturing a semiconductor integrated circuit, and minute adherent substances are removed by enhancing the purities and cleanliness levels of the chemical solution and pure water in that method. However, this method poses problems such as consumption of considerable amounts of chemical solution and pure water. In recent years, other cleaning methods such as a method of blowing minute water droplets at ultrafast speed, a method of irradiation with pulsed light, and a method of irradiation with radicals have also been proposed. When the adherent substances are, for example, organic compounds containing carbon as their major component, a method of removing adherent substances by irradiation with ultraviolet light in an atmosphere containing oxygen is also available (Japanese Patent Laid-Open No. 2000-88999). In this case, active oxygen is generated upon irradiating oxygen molecules with ultraviolet light, and reacts with carbon atoms in the adherent substances to form carbon dioxide, thereby removing the carbon dioxide.
In the conventional method, ultraviolet light is absorbed by oxygen in the atmosphere, so a sufficient removal rate cannot be obtained, thus requiring a long time to remove adherent substances. Prolongation of the time taken to remove adherent substances is undesirable because this increases the downtime of the exposure apparatus. For this reason, a demand has arisen for a technique that efficiently removes substances adhering to the surface of a soft X-ray optical element. Although a soft X-ray optical element has been described above, even adhesion of carbon onto other optical elements such as an optical element which uses ultraviolet light may also pose problems such as a decrease in reflectance or transmittance. In an exposure apparatus (a charged particle beam exposure apparatus or a charged particle beam drawing apparatus) which uses a charged particle beam as well, substances such as carbon may adhere to an optical element (an element which has the same effect on a charged particle beam as that exerted on a light beam by an optical element). The adhering substances may be charged and thereupon hinder appropriate exposure or drawing.